I lightend my naturally dark brown hair to try and get it a light brown.
After doing so TWICE I found that my hair was bright orange but my roots are a yellowy blonde.
I want to die my whole hair a "Violet Vision" colour.
Is this safe? Will I get a green tint to it??

You won't get a green tint (that come from darkening bleached hair). I'm a bit surprised that it's orange after doing it twice. Did you use hair bleach or just a single-process haircolor? Did you try to bleach already-dyed hair? If you did, and if it's stopping at orange, you probably won't get past that.

You said "naturally dark brown hair". That suggests that your hair was natural and not already-colored. If that's true, then you undoubtedly used a single-process haircoloring and not a true hair bleach. The reason the roots were lighter is because of the heat from the scalp -- it accelerated the lightening near the roots.

If this is true, then you need to get some hair bleach. There are many brands. Clairol 7th stage bleach (comes with activator packets purchased separately); L'Oreal Super Blonde; Clairol Born Blonde (there are three formulas), etc. Lighten the hair to pale yellow. Put the lightening mixture on the roots only after the ends have been bleached almost as light as you want them.

Once you get that, you can go with the violet/purple color.